U.S. Pat. No. 4,666,867 disclosed the preparation of glass microfoams and gas-ceramics (foamed glass-ceramic bodies) wherein the foams consisted of minute hydrogen-containing cells, those cells being produced through the inclusion in the starting batch of a hydrogen-generating agent selected from the group consisting of ammonia or ammonium salts and/or a combination of amines and/or carbohydrates and/or hydrocarbons with phosphates. Three distinct composition areas suitable for the production of the disclosed gas-ceramics were provided, viz.:
(a) compositions within the system SiO.sub.2 -Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -RO-R.sub.2 O consisting essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 25-65 RO 0-15 Al.sub.2 O.sub.3 15-35 R.sub.2 O 0-20 B.sub.2 O.sub.3 12-35 RO + R.sub.2 O 0.15-20 P.sub.2 O.sub.5 0-10 ______________________________________
wherein RO is selected from the group consisting of MgO, CaO, SrO, BaO, and ZnO, and R.sub.2 O is selected from the group consisting of alkali metal oxides;
(b) compositions within the SiO.sub.2 -Al.sub.2 O.sub.3 (B.sub.2 O.sub.3)-P.sub.2 O.sub.5 -Li.sub.2 O-[ZrO.sub.2 (TiO.sub.2)] system consisting essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 40-50 ZrO.sub.2 0-10 Al.sub.2 O.sub.3 15-35 B.sub.2 O.sub.3 0-15 P.sub.2 O.sub.5 10-25 TiO.sub.2 0-5; and Li.sub.2 O 1-7 ______________________________________
(c) compositions within the SiO.sub.2 -B.sub.2 O.sub.3 -P.sub.2 O.sub.5 -[RO] system consisting essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 10-65 B.sub.2 O.sub.3 5-25 P.sub.2 O.sub.5 15-60 ______________________________________
which may optionally contain 0.5-10% MgO+CaO+ZnO consisting of 0-5% MgO, 0-5% CaO, and 0-10% ZnO.
It was observed that the gas-ceramics prepared from the above (a) and (c) group compositions exhibited an integral, dense glassy skin which not only imparted a pleasing aesthetic appearance to the foamed articles, but also enhanced the mechanical strength thereof and allowed the articles to be easily cleaned, since it barred the penetration of foreign particles into the articles. Where desired, it enabled the articles to be still further strengthened through thermal tempering.
It was also observed that the compositions of the above (c) group yielded the most uniform foams with bubbles (cells) of the smallest dimensions. Hence, cell diameters ranged between about 1-100 microns, with the preferred products having cells with diameters varying between about 5-20 microns. The article densities averaged about 1 g/cm.sup.3. The inclusion of the MgO, CaO, and/or ZnO in the base SiO.sub.2 -B.sub.2 O.sub.3 -P.sub.2 O.sub.5 system was posited to inhibit restriction of cell expansion resulting from the crystallization in situ of BPO.sub.4, thereby enabling the generation of a very minute, uniform cell size and the development of a smooth, glassy, monolithic skin on the foam bodies.
As can be appreciated, the use of a hydrogen-containing compound in the original glass forming batch (ammonium acid phosphate is stated to be the preferred batch ingredient) raises problems and creates hazards during the melting operation. Accordingly, the primary objective of the present invention was to prepare gas-ceramic articles having a microstructure of the type described in U.S. Pat. No. 4,666,867, but which would not require the inclusion of a hydrogen-containing batch ingredient. A further objective was to expand the list of ingredients which, when added to the aforementioned oxide batches, would produce similar gas-ceramic articles.